Optical recording medium

ABSTRACT

An optical recording medium comprises an optical recording layer provided on a substrate, a protective material and a bond layer for bonding the optical recording layer to the protective material, the bond layer having a thermoplastic adhesive comprising at least one of (A) at least one selected from an ethylene-acrylic acid copolymer and an ethylene-acrylate ester copolymer and (B) an ethylene-maleic anhydride-acrylic acid terpolymer; and (C) a tackifier.

This application is a continuation of application Ser. No. 229,966,filed Aug. 9, 1988, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to media for optically recording and reproducinginformation.

2. Related Background Art

Heretofore, magnetic materials have been mainly used as materials forrecording media, for example, credit cards floppy disc, etc. Magneticrecording media are advantageous that writing and reading of informationcan be readily carried out, but are problematic in that the recordedinformation can be readily altered and high density recording cannotcarried out. In order to solve these problems and efficiently handlingvarious kinds of information, methods for optically recordinginformation with an optical recording medium have been proposed. In thisconnection, optical recording media, recording and reproducing systems,recording and reproducing apparatus, etc. have been also proposed. Forexample, an optical recording medium having a recording layer comprisinga gelatin matrix and silver particles dispersed in the gelatin matrixhas been proposed as a medium for optically recording and reproducinginformation. Furthermore, the heat mode recording material which canrecord information by spotwise irradiating a recording layer containingthe recording material with an energy beam such as laser beam, etc.,thereby partially changing the state of the recording layer has beenalso proposed. These recording materials require no developing treatmentafter the writing of information, and thus provide DRAW (direct readafter write) media which are expected to have applications as disc orcard recording materials due to their high density recording capabilityand their information adding capability.

A film comprising an organic coloring matter whose physical propertiescan be changed with light of relatively long wavelength is free from theaforementioned problems, is non-toxic and can be formed by coating, thatis, at a low product cost. Thus, applications of the film to an opticaldisk or card have been extensively studied and proposed.

Optical recording media now used in offices, for example, optical disks,etc. generally have a hollow structure, but pocketable optical recordingmedia, for example, optical cards are desirably made as thin aspossible. Although a tightly sealed structure is desirable for thispurpose, the tight sealing leads to the following problem.

In case of the optical recording layer comprising an organic coloringmatter, a material used in a bond layer laid on the recording layer forbonding it to a protective substrate may chemically attack the recordinglayer, resulting in a decrease in the recording contrast and a failurein information reproduction.

The optical recording media used in offices are preserved at constanttemperature and humidity or encased with jackets, whereas pocketableoptical recording media are actually exposed to severe serviceconditions, for example, severe temperature and humidity conditions, andalso have many chances for bending, resulting in occurrence of peelingat the bond layer.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical recordingmedium of good heat and cold resistance, free from peeling under highand low temperature conditions.

Another object of the present invention is to provide an opticalrecording medium free from deterioration of the recording layer due toan adhesive and free from peeling due to repeated bending.

According to the present invention, there is provided an opticalrecording medium which comprises an optical recording layer provided ona substrate, a protective material and a bond layer for bonding theoptical recording layer to the protective material, the bond layerhaving a thermoplastic adhesive comprising at least one of (A) at leastone selected from an ethylene-acrylic acid copolymer and anethylene-acrylate ester copolymer and (B) an ethylene-maleicanhydride-acrylic acid terpolymer; and (C) a tackifier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing one embodiment of theoptical recording medium of the present invention.

FIGS. 2A and 2B show a bending durability test procedure applied to theoptical recording medium of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present optical recording medium will be described in detail below,referring to the accompanying drawings.

FIG. 1 is a schematic cross-sectional view showing one embodiment of anoptical card as an example of the present optical recording medium,where an optical card of the present invention comprises a transparentsubstrate 1 having track grooves 5, an optical recording layer 2, laidon the track groove side of the transparent substrate 1, and aprotective material 4 bonded to the optical recording layer 2 through abond layer 3.

The bond layer 3 of the present invention comprises a thermoplasticadhesive containing a tackifier (hereinafter referred to as compound C)and at least one of ethylene-acrylic acid copolymer or ethylene-acrylateester copolymer hereinafter referred to as compound A), andethylene-maleic anhydride-acrylic acid terpolymer hereinafter referredto as compound B). It is preferable that the thermoplastic adhesive forthe bond layer contains 60 to 90% by weight of the total weight ofcompounds A and B and 10 to 20% by weight of compound C. Thethermoplastic adhesive may also contain 1 to 10% by weight of anadditive. The additive includes, for example, a plasticizer, a fillerfor extending purpose, anti-blocking, odor improvement, or reduction inthe shrinkage, an anti-oxidant for improving the heat stability, and anelastomer for improving the softness.

Compound C as a tackifier includes, for example, terpene resin,terpene-phenol resin, hydrogenated terpene resin and terpene-phenolresin, petroleum resin of C₅ or C₉, rosin resin and its derivative, lowmolecular weight rubber, etc. Use of at least one of terpene resin,terpene-phenol resin and hydrogenated terpene resin and terpene-phenolresin is preferable from the viewpoint of lowering the softening point.

The thermoplastic adhesive for the bonding of the optical recordinglayer is desirably those that can give a satisfactory bonding strengthat a low bonding temperature, when the heat resistance of the opticalrecording layer is taken into account. The thermoplastic adhesive forthe bond layer 3 of the present invention has a softening point as lowas 40°-60° C. and is particularly effective for an optical recordinglayer containing an organic coloring matter susceptible to heatdeterioration and has a good cold resistance and a good adhesion to apolycarbonate substrate.

The thermoplastic adhesive, as in a sheet form, can bond a transparentsubstrate provided with an optical recording layer to a protectivematerial, or a protective material onto which the thermoplastic adhesiveis heat sealed in advance or a protective material provided with thethermoplastic adhesive through another adhesive can be bonded to thetransparent substrate provided with the optical recording layer. Thethickness of the bond layer is not particularly limited, but is usually5 to 200 μm, preferably 10 to 80 μm.

The optical recording layer 2 preferably has a large difference betweenthe reflectivity at the pits, etc. as the recorded parts and thereflectivity at unrecorded parts approximately at the wavelength of alight source for recording and/or reproduction, for example, when thewavelength of reproduction energy beam such as semi-conductor beam is650 nm or more, particularly 700-900 nm. It is also necessary forwriting that the absorption is in the aforementioned wavelength range.Furthermore, it is preferable that the energy for changing thereflectivity by irradiation with a recording energy beam is low.

An optical recording layer, which can satisfy these requirements, ispreferably a layer formed from a film comprising an organic coloringmatter, particularly a polymethine-based organic coloring matterrepresented by the following general formula (I). ##STR1##

An excellent optical recording medium without deterioration of theproperties of the recording layer containing a polymethine-basedcoloring matter very susceptible to attacking of an adhesive or heat,though it has distinguished recording-reproduction characteristics asmentioned above, can be obtained particularly by using the presentadhesive. Further, each of these polymethine-based coloring matters hasa high absorption coefficient in a semi-conductor laser wavelengthregion of about 800 nm and a good solubility and thus is preferable inthe present invention. As the polymethine-based coloring matter, thosedisclosed in Japanese Patent Application Kokai (Laid-open) No. 58-219090can be used.

A polymethine-based organic coloring matter represented by the generalformula (I) will be described in detail below:

In the general formula (I), R₁, R₂, R₃ and R₄ each represent a hydrogenatom or an alkyl group, for example, methyl, ethyl, n-propyl,iso-propyl, n-butyl, sec-butyl, iso-butyl, t-butyl, n-amyl, t-amyl,n-hexyl, n-octyl, t-octyl, etc., or further other alkyl groups, forexample, a substituted alkyl group such as 2-hydroxyethyl,3-hydroxypropyl, 4-hydroxybutyl, 2-acetoxyethyl, carboxymethyl,2-carboxyethyl, 3-carboxypropyl, 2-sulfoethyl, 3-sulfopropyl,4-sulfobutyl, 3-sulfate propyl, 4-sulfate butyl,N-(methylsulfonyl)-carbonylmethyl, 3-(acetylsulfamyl) propyl,4-(acetylsulfamyl)butyl, etc., a cyclic alkyl group such as cyclohexyl,etc., an alkenyl group such as vinyl, allyl, propenyl, butenyl,pentenyl, hexenyl, heptenyl, octenyl, dodecenyl, prenyl, etc., anaralkyl group such as benzyl, phenethyl, α-naphthylmethyl,β-naphthylmethyl, etc., and a substituted aralkyl group such ascarboxybenzyl, sulfobenzyl, hydroxybenzyl, etc. Furthermore, R₁, R₂, R₃and R₄ each represent a substituted or unsubstituted aryl group such asphenyl, naphthyl, tolyl, xylyl, methyphenyl, dimethoxyphenyl,trimethoxyphenyl, ethoxyphenyl, dimethylaminophenyl, diethylaminophenyl,dipropylaminophenyl, dibenzylaminophenyl, diphenylaminophenyl, etc., asubstituted or unsubstituted heterocyclic group such as pyridyl,quinolyl, lepidyl, methylpyridyl, furyl, thienyl, indolyl, pyrrolyl,carbazolyl, N-ethylcarbazolyl, etc., and a substituted or unsubstitutedstyryl group such as styryl, methoxystyryl, dimethoxystyryl,trimethoxystyryl, ethoxystyryl, dimethylaminostyryl, diethylaminostyryl,dipropylaminostyryl, dibenzylaminostyryl, diphenylaminostyryl,2,2-diphenylvinyl, 2-phenyl-2-methylvinyl,2-(dimethylaminophenyl)-2-phenylvinyl,2-(diethylaminophenyl)-2-phenylvinyl,2-(dibenzylamino-phenyl)-2-phenylvinyl, 2,2-di(diethylaminophenyl)vinyl,2,2-di(methoxyphenyl)vinyl, 2,2-di(ethoxyphenyl)vinyl,2-(dimethylaminophenyl)-2-methylvinyl,2-(diethylamino-phenyl)-2-ethylvinyl, etc. n is 0.1 or 2.

X.sup.⊖ represents an anionic group, for example, anions such aschloride ion, bromide ion, iodide ion, perchlorate ion, benzenesulfonateion, p-toluenesulfonate ion, methylsulfate ion, ethylsulfate ion,propylsulfate ion, tetrafluoroborate ion, tetraphenylborate ion,hexafluoroarsenate ion, hexafluorophosphate ion, benzenesulfinate ion,acetate ion, trifluoroacetate ion, propionacetate ion, benzoate ion,oxalate ion, succinate ion, malonate ion, oleate ion, stearate ion,citrate ion, monohydrogen diphosphate ion, dihydrogen monophosphate ion,pentachlorostanate ion, chlorosulfonate ion, fluorosulfonate ion,trifluoromethanesulfonate ion, hexafluoroantimonate ion, molybdate ion,tungstenate ion, titanate ion, zirconate ion, etc.

Typical examples of the polymethine-based organic coloring matterrepresented by the general formula (I) are given below: ##STR2##

These polymethine-based organic coloring matters can be readilysynthesized according to the processes disclosed by Bernard S. Wildi etal. [J. Am. Chem. Soc. 80 3772-3777(1958)], H. Schmidt et al. [LiebigAnnalen der Chemie 623, 204-216]and R. Wilzinger et al. [Helv. Chim.Acta 24 369].

Any one of the aforementioned polymethine-based organic coloring matterscan be used in the present invention. As a result of further studies inview of such requirements that the solubility must be high,crystallization of a film obtained after the coating and drying must behard to take place, and the pigment must be hard to undergo changes in,for example, reflectivity, transmissivity, crystallization, etc. under aconstant temperature and humidity atmosphere (50° C.; 90% RH), apolymethine-based organic coloring matter of D-1, given by the followingformula (II) has been found excellent. ##STR3##

In forming of an optical recording layer, other dyes, for example, ametal chelate complex-based dye such as diazulene-based, azulene-based,polymethine-based, pyrilium-based, squalium-based, croconium-based,triphenylmethane-based, xanthene-based, anthraquinone-based,cyanine-based, phthalocyanine-based, dioxazine-based,tetrahydrocholine-based, triphenothiazine-based, and phenanthrene-baseddyes or metal or metal compound such as Al, Te, Bi, Sn, In, Se, SnO,TeO₂, As, Cd, etc. may be mixed or dispersed with the polymethine-basedorganic coloring matter represented by the general formula (I) or laidon a layer containing the polymethine-based organic compound representedby the general formula (I).

Such dyes may be contained in a binder in a dispersion or solutionstate. As the binder, for example, cellulose esters such asnitrocellulose, cellulose phosphate, cellulose sulfate, celluloseacetate, cellulose propionate, cellulose butyrate, cellulose myristate,cellulose palmitate, cellulose acetate-propionate, celluloseacetate-butyrate, etc., cellulose ethers such as methylcellulose,ethylcellulose, propylcellulose, butylcellulose, etc., polystyrene,polyvinyl chloride, etc. can be used.

When the organic coloring matter is used for the optical recordinglayer, an organic sovlent is used. The organic solvent applicable to thecoating depends upon application state, that is, a dispersion state or asolution state, but generally includes an alcohol such as methanol,ethanol, isopropanol, diacetonealcohol, etc., a ketone such as acetone,methylethylketone, cyclohexanone, etc., an amide such asN,N-dimethylformamide, N,N-dimethylacetamide, etc., a sulfoxide such asdimethyl sulfoxide, etc., and an ether such as tetrahydrofuran, dioxane,ethyleneglycolmonomethyl ether, etc. The optical recording layer can beformed according to a known coating method, for example, by dip coating,spray coating, spinner coating, bar coating, blade coating, rollcoating, curtain coating, etc.

The optical recording medium of the present invention can have anunderlayer between the optical recording layer and the substrate inorder to (a) improve adhesion, (b) serve as a barrier against water or agas, (c) improve the preservation stability of the optical recordinglayer, (d) improve reflectivity, (e) protect the substrate from thesolvent and (f) form pregrooves.

For purpose (a), polymeric materials, for example, various materialssuch as ionomer resin, amide-based resin, vinyl-based resin, naturalpolymers, silicones, liquid rubber, etc. or various substances such assilane coupling agent, etc. can be used.

For purposes (b) and (c), inorganic compounds such as SiO₂, MgF₂, SiO,TiO₂, ZnO, TiN, SiN, etc. or metals or semi-metals such as Zn, Cu, Si,Ni, Cr, Ge, Se, Cd, Ag, Al, etc. can be used besides the above-mentionedpolymeric materials.

For purpose (d), metals such as Al, Ag, etc. or an organic film having ametallic luster, for example, films of diazulene dye, polymethine dye,etc. can be used.

For purposes (e) and (f), an ultraviolet-curable resin, a thermosetresin, a thermoplastic resin, etc. can be used.

The thickness of the underlayer is 50 Å to 100 μm, preferably 200 Å to30 μm.

The underlayer may contain a stabilizer, a dispersing agent, a flameretardant, a lubricant, an antistatic agent, a surfactant, aplasticizer, etc.

The transparent substrate 1 preferably has less inconvenience in theoptical recording and reproduction, and can be formed from, for example,acrylic resin, polyester resin, polycarbonate resin, vinyl-based resin,polysulfone resin, polyimide-based resin, polyacetal resin, polyolefinresin, polyamide resin, cellulose derivatives, etc.

In the present invention, track grooves 5 can be formed on thetransparent substrate 1, in case that the transparent substrate isformed from a thermoplastic resin, by heat transfer of a stamper patternthrough injection molding or hot press molding at a temperature higherthan the melting point, or photo-transfer of a stamper pattern byapplying a photo-curable resin composition to the transparent substratetightly placing a stamper pattern on the resin composition andirradiating the exposed resin composition with ultraviolet ray or thelike, thereby curing the exposed photo-curable resin composition.

In case of the heat transfer, track grooves are indented on thetransparent substrate itself, whereas in case of the photo-transfer thetrack grooves are formed on another material laid by adhesion onto thetransparent substrate.

Besides the transfer of a stamper pattern, track grooves can be alsoformed on a transparent substrate by forming a resist film on atransparent resin substrate with an etchable light-transmissible film ofa desired thickness, irradiating the resist film with a light through aphoto mask having a track groove pattern, forming a resist pattern bydevelopment, and etching the light-transmissible film by means of theresist pattern as a mask.

The protective material 4 can be formed from any material so long as itcan be applied for an ordinary protective material, and can be formedspecifically from polyvinyl chloride, vinyl chloride-vinyl acetatecopolymer, polyvinylidene chloride, acrylic polymer such as polymethylmethacrylate, etc., polystyrene, polyvinylbutyral, acetylcellulose,styrene-butadiene copolymer, polyethylene, polypropylene, polycarbonate,epoxy resin, acrylonitrile-butadiene-styrene copolymer, etc. A metalsheet of iron, stainless steel, aluminum, tin, copper, zinc, or thelike, synthetic paper, paper, etc. can be used as the material for theprotective material. Furthermore, a laminate of the aforementioned sheetmaterials can be applied for the protective material 4. The opaqueprotective material 4 may be pretreated by corona discharge treatment,plasma treatment, primer treatment, or the like to improve the adhesion,if required.

The present optical recording medium can record information byirradiation with a gas laser such as an argon laser (oscillationwavelength : 488 nm), a helium-neon laser (oscillation wavelength : 633nm), a helium-cadmium laser (oscillation wavelength : 442 nm), or thelike, and also can record information by irradiation with a SHG devicebeam of half-wavelength of the laser beam.

Recording by irradiation with a laser beam having an oscillationwavelength in the near infrared region such as a laser having awavelength of 650 nm or more, particularly a gallium-aluminum-arsenicsemi-conductor laser (oscillation wavelength : 830 nm) is preferable.For reading, the afore-mentioned laser beam can be used. Writing andreading can be made with laser beams of same wavelength or with laserbeams of different wavelengths.

As described above, the bond layer of the present optical recordingmedium is formed from a thermoplastic adhesive comprising at least oneof (A) at least one of an ethylene-acrylic acid copolymer and anethylene-acrylate ester copolymer and (B) an ethylene-maleic anhydrideacrylic acid terpolymer; and (C) a tackifier as mentioned above, and hasdistinguished adhesion and flexibility without any deterioration of theorganic coloring matter in the optical recording layer. Thus, thepresent invention can provide an optical recording medium free frompeeling while used and from any decrease in the sensitivity.

Conventional optical cards comprising a metallic thin film as an opticalrecording layer have such problems in that optical card sheets are lessamenable to stamping for finishing the card sheet into card forms, theoptical cards have a low bending durability, and the optical recordinglayer is liable to crack due to the hardness of the metallic filmitself, whereas the present optical recording medium has no suchproblems since a thin film containing the aforementioned organiccoloring matter is used as the optical recording layer.

The present invention will be described in detail below, referring toExamples, but will not be limited only thereto.

EXAMPLE 1

Track grooves with a groove width of 3 μm, a track pitch of 12 μm and adepth of 0.2 μm were formed on a polymethyl methacrylate casting platehaving a thickness of 0.4 mm as a transparent resin substrate by hotpress. A diacetone-alcohol solution containing 4 % by weight of the samepolymethine-based coloring matter represented by the formula (II) as theafore-mentioned D-1 was applied to the track groove surface on theformed transparent resin substrate by bar coating, whereby thetransparent substrate provided with an optically recording layer to athickness of 1,000 Å on the track groove surface was obtained.

A polymethyl methacrylate casting plate having a thickness of 0.3 mm asa protective material was laid on the transparent substrate providedwith the optical recording medium through a bond layer of athermoplastic adhesive (softening point : 43°-51° C.) comprising 60% byweight of ethylene-acrylic acid copolymer, 17% by weight ofethylene-maleic anhydride-acrylic acid terpolymer, 20% by weight ofterpene resin and 3% by weight of an additive and having a thickness of50 μm, so that the optical recording layer can contact the thermoplasticadhesive, and bonded thereto by pressing with hot rolls at a surfacetemperature of 110° C. Then, an optical card, 54.0 mm wide and 85.5 mmlong, was prepared therefrom by stamping out with a hollow rectangularstamping edge.

EXAMPLE 2

An optical card was prepared in the same manner as in Example 1, exceptthat a coloring matter of aforementioned D-8 was used as thepolymethine-based coloring matter and a thermoplastic adhesive(softening point : 41°-50° C.) comprising 45% by weight ofethylene-acrylic acid copolymer, 40% by weight of ethylene-maleicanhydride-acrylic acid terpolymer and 15% by weight of hydrogenatedterpene resin was used.

EXAMPLE 3

Track grooves with a groove width of 3 μm, a track pitch of 12 μm and adepth of 0.2 μm were formed on a polymethyl methacrylate casting platehaving a thickness of 0.4 mm as a transparent resin substrate by hotpress, and a diacetone-alcohol solution containing 4% by weight of thesame polymethine-based coloring matter represented by the foregoingformula (II) as D-1 was applied to the groove surface by bar coating,whereby a transparent substrate provided with an optical recording layerto a thickness of 1,000 Å on the groove surface was obtained.

A polymethyl methacrylate casting plate having a thickness of 0.3 mm asa protective material was laid on the transparent substrate providedwith the optical recording layer through a bond layer of a thermoplasticadhesive (softening point : 46°-58° C.) comprising 70% by weight ofethylene-acrylic acid copolymer, 20% by weight of terpene resin and 10%by weight of an additive and having a thickness of 50 μm so that theoptical recording layer can contact the thermoplastic adhesive, andbonded thereto by pressing with hot rolls at a surface temperature of80° C. Then, an optical card, 54.0 mm wide and 85.5 mm long was stampedout with a hollow rectangular edge.

EXAMPLE 4

An optical card was prepared in the same manner as in Example 3, exceptthat polycarbonate casting plates were used in place of the polymethylmethacrylate casting plates used as the transparent resin substrate andthe protective material.

EXAMPLE 5

An optical card was prepared in the same manner as in Example 4, exceptthat a thermoplastic adhesive comprising 65% by weight of ethylene-ethylacrylate copolymer, 20% by weight of ethylene-acrylic acid copolymer,10% by weight of terpene-phenol resin and 5% by weight of an additivewas used.

EXAMPLE 6

An optical card was prepared in the same manner as in Example 4, exceptthat the components of the thermoplastic adhesive were changed to 75% byweight of ethylene-ethyl acrylate copolymer, 15% by weight of terpeneresin and 10% by weight of additive.

Comparative Example 1

An optical card was prepared in the same manner as in Example 1, exceptthat a polyester-based adhesive, Kemitto (trademark of a product made byToray K.K. Japan) was used as a thermoplastic adhesive in place of theadhesive of Example 1.

Comparative Example 2

An optical card was prepared in the same manner as in Example 1, exceptthat an amide-based adhesive, Daiamide, (trademark of a product made byDaicel Kagaku Kogyo K.K., Japan) was used as a thermoplastic adhesive inplace of the adhesive of Example 1.

Comparative Example 3

An optical card was prepared in the same manner as in Example 1, exceptthat a thermoplastic adhesive comprising 70% by weight ofethylene-acrylic acid copolymer and 30% by weight of ethylene-maleicanhydride-acrylic acid terpolymer was used in place of the adhesive ofExample 3.

The optical recording media prepared in Examples 1 to 6 and comparativeExamples 1 to 3 were evaluated through the following tests:

(1) Contrast ratio

The optical card was scanned with a laser beam with a wavelength of 830nm, a laser power of 3.5 mW, a beam spot diameter of 3.2 μm, and a pulsewidth of 50 μm at a speed of 60 mm/s to make recording, and a contrastratio was determined. Contrast ratio is a value obtained by subtractinga ratio of the reflectivity at recorded parts to the reflectivity atunrecorded parts from 1.

(2) Heat resistance

The optical card was dipped in hot water at 60° C. to observe whetherpeeling took place at the bond layer or not.

(3) Bonding strength

The optical card not subjected to the heat resistance test was cut topieces, 25 mm width, which were subjected to a T-type peeling test todetermine the bonding strength at room temperature (25° C.) and -5° C.

(4) Bending test

The untested optical card was supported at the short side edges as shownin FIG. 2A and reciprocally bent 250 times in the lateral directionuntil the center part of the card was extended over a distance of 20 mmas shown by A in FIG. 2B, and then the optical card was supported at thelong side edges and reciprocally bent 250 times in the lateral directionuntil the center part of the card was extended over a distance of 10 mmas shown by A to observe whether peeling took place at the bond layer ornot.

The results are shown in the following Table.

                  TABLE 1                                                         ______________________________________                                        Con-                 Bonding strength                                         trast      Heat      (kg/25 mm)    Bending                                    ratio      resistance                                                                              25° C.                                                                          -5° C.                                                                        test                                     ______________________________________                                        Example 1                                                                             0.49   no change 0.3    0.25   no peeling                             Example 2                                                                             0.45   "         0.4    0.30   "                                      Example 3                                                                             0.46   "         0.4    0.35   "                                      Example 4                                                                             0.47   "         0.3    0.29   "                                      Example 5                                                                             0.48   "         0.4    0.37   "                                      Example 6                                                                             0.46   "         0.4    0.35   "                                      Comp.   0.45   Peeled    0.2    less   "                                      Example 1      after 72         than 0.1                                                     hours                                                          Comp.   *      Peeled    0.7    0.51   "                                      Example 2      after 72                                                                      hours                                                          Comp.   0.48   no change less   --     Peeled                                 Example 3                than 0.1                                             ______________________________________                                         *The adhesive attacked the coloring matter in the optical recording layer     and recording was impossible to make.                                    

We claim:
 1. An optical recording medium which comprises an opticalrecording layer provided on a substrate, a protective material and abond layer for bonding the optical recording layer to the protectivematerial, the bond layer having a thermoplastic adhesive comprising: (A)a tackifier and at least one of (B) an ethylene-acrylic acid copolymer,an ethylene-acrylate ester copolymer, or an ethylene-maleicanhydride-acrylic acid terpolymer.
 2. An optical recording mediumaccording to claim 1, wherein the tackifier is at least one of terpeneresin, terpene-phenol resin and their hydrogenated resin.
 3. An opticalrecording medium according to claim 1, wherein the optical recordinglayer contains a polymethine-based organic coloring matter.
 4. Anoptical recording medium according to claim 3, wherein thepolymethine-based, organic coloring matter is a compound represented bythe following general formula (I): ##STR4## where R₁, R₂, R₃ and R₄ eachrepresent a hydrogen atom, an alkyl group, a substituted alkyl group, acyclic alkyl group, an alkenyl group, an aralkyl group, a substitutedaralkyl group, an aryl group, a substituted aryl group, a styryl group,a substituted styryl group, a heterocyclic group or a substitutedheterocyclic group; n is 0, 1 or 2; and X.sup.⊖ represents an anionicgroup.
 5. An optical recording medium according to claim 3, wherein thepolymethine-based, organic coloring matter is a compound represented bythe following formula (II): ##STR5##
 6. An optical recording mediumaccording to claim 1, wherein the thermoplastic adhesive contains from60 to 90% by weight of the total weight of said ethylene-acrylic acidand ethylene-acrylate ester copolymers, and said ethylene-maleicanhydride-acrylic acid terpolymer, and from 10 to 20% by weight of saidtackifier.
 7. An optical recording medium according to claim 1, whereinthe bond layer has a thickness of 5 to 200 μm.
 8. An optical recordingmedium according to claim 1, wherein the optical bond layer has athickness of 10 to 80 μm.
 9. An optical recording medium according toclaim 1, wherein the optical recording layer has a thickness of 300 Å to1500 Å.
 10. An optical recording medium according to claim 9, whereinthe optical recording layer has a thickness of 700 Å to 1300 Å.
 11. Anoptical recording medium according to claim 1, wherein the opticalrecording medium is a card-type optical recording medium.
 12. An opticalrecording medium according to claim 1, wherein an underlayer is providedbetween the optical recording layer and the substrate.